Coupling a horn or an acoustic pipe to the front of a speaker unit in order to guide a sound wave or waves generated by the speaker unit to the opening of an acoustic pipe is a method which has advantages over a case without a horn or an acoustic pipe. For example, the sound can be conveyed to a specific direction, or the output of a larger sound pressure is obtainable. Because of these features, the method has been widely used in many such applications.
FIG. 10 shows a conceptual construction of a television receiver set comprising such a speaker system. Where, the listed numerals identify the following elements: 1 denotes an acoustic pipe, 2 a speaker unit, 3 a cabinet of television set, 4 a cathode ray tube, 5 a back cabinet. Symbol "a" represents a radiation sound wave of a front side of a television set, and symbol "B" represents a radiation sound wave of a back side of television set.
With a television receiver set having the above described constitution (or construction), its operation is described hereunder. A sound wave generated by speaker unit 2 goes to an acoustic pipe 1, which is coupled to the front of the speaker unit, has an oblong shaped, approximately rectangle, opening. The entire speaker system is constructed to take a shape running along cathode ray tube 4 and cabinet 3 of television set within the inside of the television receiver. The sound wave is guided to the front through the narrow space to be radiated as the radiation sound wave of a front side "all from an opening of cabinet 3 of the television set, and at the same time a sound wave is radiated at the back of speaker unit 2 as the radiation sound wave of back side B to the inside of back cabinet 5.
The above described constitution enables one to make a television receiver set small and slim. Now in the following, the speaker system comprising the speaker unit 2 and the acoustic pipe 1 is described in detail referring to FIG. 11 and FIG. 12.
FIG. 11 shows a perspective view of prior art speaker system, and FIG. 12 a cross sectional side view of the speaker system. In these FIGURES, the listed numerals represent the following elements: 6 denotes a sound path through which a sound wave generated by speaker unit 2 proceeds, 7 a hole for absorption sound facing sound path 6, 10 a resonance absorption section comprising a tube 8 with the hole 7 as its one end and a cavity 9 which is coupled through with the other end of tube 8, and 11 an opening through which a sound wave is radiated.
With the above described constitution, the operation of a prior art speaker system is described as follows. When an input signal reaches speaker unit 2, a sound wave proceeds along the sound path 6 of the acoustic pipe 1 so as to be radiated through opening 11 of acoustic pipe 1.
In the mean time, however, because of a substantial shift in the acoustic impedance is caused at the opening 11 of the acoustic pipe 1, a part of the sound wave is reflected and returned to the sound path 6. This creates a standing wave in accordance with the length of acoustic pipe 1, which enables the response frequency characteristics to have their peak at the mid-range frequency.
In order to remove the peak in mid-range frequency caused by the standing wave, the above described prior art constitution tried to absorb the peak part with the resonance absorption section 10, which comprised the tube 8 with hole 7 facing sound path 6 at its one end and the cavity 9 coupled through with the other end of tube 8. The resonance absorption section 10 has a sealed structure except that it is coupled with tube 8.
FIGS. 13 and 14 illustrate other prior art speaker systems intended to absorb such a standing wave, with FIG. 13 showing a cross sectional side view, and FIG. 14 showing a perspective view exploded. In these FIGURES, the following numerals represent the following elements: 15 denotes a speaker unit for generating sound wave, 16 an acoustic pipe coupled in the front of speaker unit 15, 17a a screw for coupling and fixing speaker unit 15 and acoustic pipe 16 together, 17 a sound path within acoustic pipe through which the sound wave proceeds, 18 a reflector board provided at both sides for guiding a sound wave generated by speaker unit 15 to an opening 16a of acoustic pipe 16, 19 an absorber disposed within a throat section 22, 20 an absorber inserted from opening 16a, 21 a bonding agent for fixing the absorber inserted from said opening 16a to acoustic pipe 16, 22 denotes a throat section having its opening facing to a sound path 17 along which the absorber 20 is to be inserted, and 23 an absorber cover for sealing a throat section 22 after disposing absorber 19, and 24 a screw for fixing the cover.
Symbol "a" indicates a radiation sound wave on the front side of a television set radiated from opening 16a of acoustic pipe 16, and symbol "B" represents a radiation sound wave of the back side of a television receiver radiated from the back of speaker unit 15.
The speaker system as described above operates in the same way as the prior art shown in FIG. 11, therefore the operational description is eliminated here. Accordingly, the following description is made on the operation for absorbing different standing waves. The standing wave (which arises according to a length specific to the acoustic pipe 16 out of a sound wave generated from the speaker unit 15 and renders the speaker system to be one having frequency characteristics of turbulent peaks and dips) can be absorbed and suppressed by the absorber 19 of the throat section 22 and the absorber 20 inserted from opening 16a within acoustic pipe 16. Thus, the flat frequency characteristics are obtainable.
In order to bring the absorber 19 of the throat section 22 to a full functioning operation, the throat section 22 is sealed with absorber cover 23 and fixed with a screw 24. Further, the reflector board 18 is disposed at both sides in order for a sound wave not to be directly absorbed and damped at throat section 22. This functions to a certain extent to control and guide the sound wave, which works for the compensation of, among others, midrange frequency characteristics. By taking advantage of this characteristic, the reproduction of natural sound is made possible even in a speaker system comprising an acoustic pipe 16 constituted to have an opening of oblong, approximately rectangle shape, and placed in a television receiver set employing the same speaker system.
However, in a prior art speaker system constituted in a way as shown in FIGS. 11 and 12, where the peak at mid-range frequency is removed by providing the resonance absorption section 10 for absorbing the peak part, the absorbed part turns out to be "a big dip." This makes the response frequency characteristics curve to have turbulent peaks and dips as indicated with numeral 53 in FIG. 4. As a result, the speech articulation deteriorates.
In a prior art constitution, as shown in FIGS. 13 and 14, the sound wave is absorbed and damped by absorber 19 of the throat section 22 and by an absorber 20 inserted from the opening 16A. To the extent more change than what is needed, the level of frequency characteristics substantially drops to a level as indicated with numeral 51 in FIG. 7. As a result, the radiation volume of radiation sound wave of back side B becomes even larger than that of radiation sound wave of front side "a," in FIG. 13, bringing about a deteriorated speech articulation.